Particle classification according to aerodynamic size can be carried out with a number of different devices, including elutriators, cyclones, centrifuges and impactors. Inertial impactors have been widely used for particle collection, mainly because of their sharp cut-off characteristics. Due to extensive theoretical work on inertial impactors, see, e.g., Marple, V. A. and Liu, B. Y. H. "Characteristics of laminar jet impactors." Environm. Sci. & Techn. 1974, 7:648-654; Marple, V. A. and Willeke, K. In "Fine Particles: Aerosol generation, measurement, sampling, and analysis." (B. Y. H Liu ed.) 1976, the performance of inertial impactors has become well understood and in many instances their characteristics can be predicted. The most important limitations of these instruments are the following: (i) particles may bounce from the collection surface upon impaction; (ii) collected particles may reentrain in the airstream; (iii) wall losses between the impactor stages may be considerable; and (iv) very large particles may break-up upon impaction, especially at high impaction velocities. See Biswas, P. and Flagan, R. C. "The particle trap impactor". J. Aer. Sci. 1988: 19:113-121. The particle bounce problem has been traditionally encountered by coating the impaction plates with a sticky material. However, carbon-containing coatings that are typically used may interfere with the measurement of carbonic compounds.
The virtual impactor provides an alternative solution to the problems of particle bounce and reentrainment. Similar to inertial impactors, virtual impactors classify particles according to aerodynamic size. In this device, a jet of particle-laden air is accelerated toward a collection probe which is slightly larger than the acceleration nozzle and which is positioned downstream from the accelerator so that a small gap exists therebetween. See Loo, B. W., Jaklevic, J. M., and Goulding, F. S. In "Fine Particles: Aerosol generation, measurement, sampling, and analysis." (B. Y. H Liu ed.) 1976. A vacuum is applied that deflects a major portion of the accelerated airstream through this small gap rather than permitting it to continue straight and thereby enter the collection probe. Particles larger than a certain threshold size (known as the cutpoint) have sufficient momentum that they cross the deflected streamlines and enter the collection probe, whereas particles smaller than the cutpoint follow the deflected streamlines. The larger particles are removed from the collection probe by the minor portion of the airstream, the quantity of which is determined by the magnitude of the vacuum applied to the major portion of the airstream and the magnitude of a vacuum applied to the minor portion. The minor flow generally contains a small fraction of fine particles also. The concentration of the larger particles in the minor flow increases by a factor of Q.sub.tot /Q.sub.min, where Q.sub.tot is the total flow entering the virtual impactor and Q.sub.min is the minor flow.
Studies have shown that essentially all of the particle mass of ambient air is of a size greater than 0.1 .mu.m. Despite the advantages described above, conventional virtual impactors with a cutpoint approaching this value have not been disclosed. Equations 1 and 2 below demonstrate the technical impediments to such an impactor. ##EQU1## wherein St is the Stokes number, a dimensionless parameter relating the jet velocity and the nozzle diameter, D.sub.j is the acceleration nozzle diameter (cm), U is the average velocity of the jet (cm/s), .rho..sub.p is the particle density (g/cm.sup.3), .mu. is the dynamic viscosity of the air (1.8*10.sup.-4 g/cm s), and C.sub.c is the Cunningham slip correction factor as given in Equation 2 ##EQU2## wherein P is the absolute pressure in cm Hg and .rho..sub.p is the particle diameter in .mu.m. Because St is a dimensionless parameter, equation (1) indicates that the square root of St provides a measure of the dimensionless particle diameter. As seen in equation (1), either a very high jet velocity (resulting in low pressures downstream of the jet nozzle) or a very small orifice must be used in order to create a virtual impactor with a 0.1 .mu.m cutpoint. For example, Biswas et al., supra, developed a virtual impactor that operated at jet velocities comparable to the sonic limit to separate particles in the order of 0.1 .mu.m. U.S. Pat. No. 5,183,481 to Felder discloses a supersonic virtual impactor with a 0.1 .mu.m cutpoint. Because of the high jet velocity and corresponding high pressure drop required for such a virtual impactor, these designs are not practical for widespread use. Small orifice impactors have not been reported; had they been constructed, they also would be impractical, as the flow rate is extremely low.
Masuda, H. and Nakasita, S. "Classification performance of a rectangular jet virtual impactor-effects of nozzle width ratio of collection nozzle to acceleration nozzle." J. Aer. Sci. 1988, 19:243-252, report the use of flow visualization techniques to examine the problem of particle losses in a slit-nozzle virtual impactor. The experiments indicated the formation of a turbulent periodic eddy in the collection probe. The eddy residence time increased with increasing collection-to-acceleration slit widths, resulting in a more unstable flow. The causes of this eddy formation were related to flow instabilities arising from the reduction of the flow velocity and the abrupt pressure recovery in the enlarged collection probe. The separation efficiency in those experiments was considerable lower than that predicted theoretically, thus resulting in a larger cutpoint than desired.
Thus a first object of the present invention is to overcome the deficiencies of the prior art and provide a virtual impactor having a 0.1 .mu.m cutpoint.
A second object of the present invention is to provide a virtual impactor having such a cutpoint that can be operated at practical airstream flow rates.
A third object of the present invention is to provide a method for separating all of the particles of an airstream from all of the gas components of the airstream.